Duplicate of A000387.

Terms

a(0) =0a(1) =0a(2) =1a(3) =0a(4) =6a(5) =20a(6) =135a(7) =924a(8) =7420a(9) =66744a(10) =667485a(11) =7342280a(12) =88107426a(13) =1145396460a(14) =16035550531a(15) =240533257860